Trump's new acting Pentagon chief just extended the mission for US troops at the US-Mexico border until the end of September

Army engineers install concertina wire on November 5, 2018, on the Anzalduas International Bridge in Texas.
US Air Force photo by Airman First Class Daniel A. Hernandez

Active-duty troops deployed to the US-Mexico border will not be leaving anytime soon, the US Department of Defense announced Monday evening.

The border mission, initially expected to wrap up on December 15, 2018, has been extended to September 30, 2019.

The cost of the deployment is expected to hit $132 million by the end of this month.

US troops deployed to the US-Mexico border will remain there until at least the end of September, the Pentagon revealed in an emailed statement Monday evening.

Acting Secretary of Defense Patrick Shanahan, who took over for former Secretary of Defense James Mattis at the beginning of the year, has approved Department of Defense assistance to the Department of Homeland Security (DHS) through September 30, 2019.

The decision was made in response to a DHS request submitted in late December.

The initial deployment, which began in October as "Operation Faithful Patriot" (since renamed "border support"), was expected to end on December 15, 2018. The mission had previously been extended until the end of January.

Thousands of active-duty troops, nearly six thousand at the operation's peak, were sent to positions in California, Texas, and Arizona to harden points of entry, laying miles and miles of concertina wire. The number of troops at the southern border, where thousands of Central American migrants wait in hopes of entering the US, has dropped significantly since the operation began.

The Department of Defense is transitioning the support provided from securing ports of entry to mobile surveillance and detection activities, according to the Pentagon's emailed statement. Troops will offer aviation support, among other services.

Shanahan has also given his approval for deployed troops to put up another 115 miles of razor wire between ports of entry to limit illegal crossings, according to ABC News.

The extension of the border mission was expected after a recent Cabinet meeting. "We're doing additional planning to strengthen the support that we're providing to Kirstjen and her team," Shanahan said, making a reference to Homeland Security Secretary Kirstjen Nielsen, Military.com reported earlier this month.

"We've been very, very closely coupled with Kirstjen," he added. "The collaboration has been seamless."

The cost of the Trump administration's border mission, condemned by critics as a political stunt, is expected to rise to $132 million by the end of this month, CNN reported recently.